Process for producing ethanol from corn dry milling

ABSTRACT

A process for producing ethanol by the conversion of carbohydrates from a corn dry milling process in which the bottoms fraction from distillation of ethanol in a conventional yeast fermentation is used in a process including a combination of biochemical and synthetic conversions. The process results in high yield ethanol production with concurrent production of high value coproducts. An acetic acid intermediate is produced from bottoms fraction, followed by conversion of the acetic acid into ethanol using esterification and hydrogenation reactions. Coproducts of the process include a high protein content solids fraction produced in the fermentation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/339,238 filed Jan. 24, 2006, which is a continuation of U.S. patentapplication Ser. No. 10/268,290, filed Oct. 9, 2002 (now U.S. Pat. No.7,074,603 issued Jul. 11, 2006), which is a continuation-in-part of U.S.patent application Ser. No. 09/720,930, filed Dec. 29, 2000 (now U.S.Pat. No. 6,509,180 issued Jan. 21, 2003), which was filed as anapplication under 35 U.S.C. 371 based on PCT Application No.PCT/US00/06498. PCT Application No. PCT/US00/06498 claims the benefit ofpriority under 35 U.S.C. 119(e) from U.S. Provisional Patent ApplicationNo. 60/124,276, filed on Mar. 11, 1999. Said U.S. patent applicationSer. No. 10/268,290 (issued U.S. Pat. No. 7,074,603) also claimspriority under 35 U.S.C. 119(e) from U.S. Provisional Patent ApplicationNo. 60/328,258, filed on Oct. 9, 2001. All of the foregoing applicationsare incorporated in their entirety by reference.

FIELD OF THE INVENTION

This invention relates to a process for the conversion of theunfermented fraction of the corn from a conventional yeast fermentationbased corn dry milling ethanol process into ethyl acetate or ethanol forfuel or chemical use. The invention uses a combination of fermentationand chemical conversion to greatly increase the yield of ethanol fromcarbohydrates compared to the current art.

BACKGROUND OF THE INVENTION

The United States is the world's largest producer of corn. US productionreached 9.5 billion bushels in 2001, greatly exceeding the production ofany other grain (National Corn Growers Association 2002). Direct use asanimal feed is the largest consuming application, accounting for 5.85billion bushels in 2001. Corn processing, either via wet milling intosweeteners, starch, ethanol and other industrial products, or via drymilling for ethanol production, accounted for 1.7 billion bushels of USconsumption in 2001, or slightly less than 18% of the crop.

Corn processing is expected to increase significantly over the nextdecade. Ethanol production has been the largest single application ofcorn processing since 1999, reflecting the recent high growth of thefuel ethanol market and the slowing growth of sweetener markets.Legislative and lobbying efforts are promoting a renewable fuelsstandard for gasoline. Most projections are for a three fold increase inethanol production, accounting for another 1.4 billion bushels of cornconsumption if no significant changes are made to existing manufacturingprocesses. Other new industrial uses for corn, such as biodegradableplastics, will also contribute to the expansion of the corn processingindustry.

A conventional corn dry milling process with the production of ethanolis illustrated in FIG. 1. Briefly, the process involves an initialpreparation step of grinding and cooking the corn 110. The resultingproduct is subjected to a step of hydrolysis 120. Then a yeastfermentation is conducted for the production of ethanol 130. The ethanolis distilled from the fermentation broth 140 and dried 150. Theremainder of the fermentation medium is then dried to produce DDGS 160.This step typically includes a solid/liquid separation 170, wherein theliquid stream is subjected to an evaporation step 180 to concentratesoluble byproducts, such as sugars, glycerol and amino acids, beforebeing recombined with the solids to be dried into DDGS.

The economics of corn processing are dependent to a great degree on thevalue of the animal feed coproducts made from the non-starch fractionsof corn. The projected expansion of the industry will likely result insevere oversupply conditions in the coproduct markets unless changes aremade. In particular with the expansion of the dry mill ethanol industrythere will be a huge increase in supply of distiller's dried grains andsolubles (DDGS). There are several limitations on the utilization ofDDGS, including limited export markets which are hampered by continuingdisputes with trading partners, and the limited market range in terms ofanimal feeding systems, essentially limited to ruminants.

It would be desirable to increase the value of the coproducts and toexpand the range of markets beyond ruminant animals. One way to do thisis to utilize the fiber fraction of the corn. Various schemes have beenderived to hydrolyze the fiber fraction of the corn for utilization as afermentation substrate. However the fiber fraction is complex and iscomposed of hemicellulose and cellulose components and yields ahydrolysate, upon enzymatic hydrolysis, for example, that is of limiteduse as a substrate in the conventional yeast based ethanol process. Thusit would be desirable to have a process that could utilize the complexmixture of components from corn fiber hydrolysate. It would also bedesirable to utilize the metabolic byproducts from the yeastfermentation such as glycerol which is a difficult material to handle inthe ethanol plant particularly in drying of DDGS. If the fiber fractionof the corn could be utilized it would increase the yield from the plantand concentrate and thus increase the value of the corn protein fractionof the corn. The increased concentration of protein with lower fibercontent is a higher value animal feed coproduct. For example, corngluten meal is a fraction of corn protein produced in corn wet millingthat has low fiber content, a higher protein concentration than DDGS,contains the pigments from the corn, and has a market value of greaterthan twice DDGS. Corn gluten meal can also be utilized in a broaderrange of markets than DDGS such as chickens and other monogastricanimals. By selectively removing the fiber fraction from DDGS, a productmore like corn gluten meal would be produced with higher value.

In addition, since there are a large number of already existing ethanolplants based on corn dry milling, and the number is increasing rapidly,it would be desirable to have a process that could be integrated withthese plants to improve the product value, increase yield and utilize tothe greatest extent possible the existing physical plant assets.

Thus, a need exists for improvements in the corn dry milling process toincrease ethanol production and to develop byproducts of higher valueand with broader markets than DDGS.

SUMMARY OF THE INVENTION

The present invention is an integrated ethanol production process thatcan be conducted in most conventional ethanol production facilitiesbased on dry corn milling. The process is for the production of ethanolfrom corn. The process includes conducting a yeast fermentation in acorn preparation to produce ethanol. The ethanol is distilled from theyeast fermentation to produce a bottoms fraction, which is thenhydrolyzed. In one embodiment, the bottoms fraction can be contactedwith a protease. The hydrolyzed bottoms fraction is separated into asolids fraction and a liquid hydrolyzate. The liquid hydrolyzate is usedas a medium for culturing a homofermentative microorganism to produceacetate and/or acetic acid. The process further includes chemicallyconverting the acetate and/or acetic acid to an ester of acetic acid,which can be ethyl acetate. In an alternative embodiment, the acetateand/or acetic acid is chemically converted to ethanol. In one embodimentof the invention, the step of hydrolyzing can include hot waterpretreatment and/or enzymatic hydrolysis (such as by contacting thebottoms fraction with xylanase and cellulase enzymes) of the bottomsfraction. In another embodiment, the fermentable fraction of the liquidhydrolysate is less than about 6% by weight.

In a further embodiment, the solids fraction comprises a majority of theprotein from the corn, and the protein content can be at least about 35%protein on a dry weight basis. The solids fraction also includes thelipid fraction from the corn. Typically, the volume of the solidsfraction is about one-half the volume of distiller's dried grains andsolubles (DDGS) produced by the same volume of corn preparation. Thesolids fraction can also be dried and in particular, the step of dryingis conducted with a dryer suitable for producing DDGS from a bottomsfraction. The step of drying can be conducted without a pre-step ofseparating and concentrating a liquid portion of the solids fraction.

In the process of the present invention, the homofermentativemicroorganism can be a homoacetogenic microorganism, such as amicroorganism is of the species Clostridium thermoaceticum orClostridium formicoaceticum. Also, the step of culturing can includeconverting by fermentation the liquid hydrolyzate into lactic acidand/or lactate, such as by a homolactic fermentation, and converting byfermentation the lactic acid and/or lactate into acetic acid and/oracetate, such as by a homoacetic fermentation.

In a further embodiment, the step of chemically converting the acetateand/or acetic acid to an ester of acetic acid can include (i)esterifying the acetic acid to an acetic acid ester in the presence ofan alcohol, and (ii) hydrogenating the acetic acid ester to ethanol andthe alcohol. The alcohol produced in step (ii) can be recycled to step(i). Alternatively, the alcohol in step (i) can be ethanol produced bythe yeast fermentation.

In another embodiment, the process can include acidifying the acetatewith carbon dioxide to produce acetic acid and calcium carbonate;esterifying the acetic acid with an alcohol to form an ester; andrecovering the ester. Further, the ester can be recovered bydistillation, such as reactive distillation capable of pushing theacidification and esterification equilibria to high conversion to thedesired ester product. Further, the ester can be converted to ethanol.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram a prior art process for the production ofethanol based on corn dry milling.

FIG. 2 is a block diagram of on embodiment of the process of the presentinvention.

FIG. 3 illustrates the metabolic pathway for the conversion of glucoseto lactate.

FIG. 4 illustrates the metabolic pathway for the conversion of glucoseto acetate.

FIG. 5 illustrates one embodiment of reactive distillation.

DETAILED DESCRIPTION OF THE INVENTION

One embodiment of the process of the present invention is a uniqueprocess for the production of ethanol based on dry milling of corn. Aconventional ethanol production process based on the dry milling of cornis shown in FIG. 1. The present invention uses the bottoms fraction fromdistillation of ethanol in the process as a carbohydrate and nitrogensource in a process that is described in more detail in U.S. patentapplication Ser. No. 09/720,930 to make an integrated ethanol productionprocess. A key feature of this approach is the production of acetic acidfrom various sugars using an anaerobic acetogenic bacterium.

A preferred embodiment of the present invention is illustrated in FIG.2. The conventional steps of corn preparation 210, hydrolysis 220,ethanol fermentation 230, distillation 240, and dehydration 250 areshown. Also shown in FIG. 2 is a bottoms fraction from the distillationis subjected to a pretreatment and second hydrolysis step 255, utilizingcellulase and xylanase enzymes, mainly to hydrolyze the fiber fractionof the bottoms. The resulting hydrolyzate is sent to a solid/liquidseparation 260. The solid fraction from the separation is then dried asa high protein byproduct 265. The liquid fraction is used in afermentation medium, in this particular embodiment, for the productionof acetic acid 270. The acetic acid is then converted to ethyl acetate275.

In an alternative embodiment, the hydrolysis step 255 includes the useof protease enzymes. In this case a majority of the solid fraction ofthe beer still bottoms stream is converted to soluble componentsincluding sugars and amino acids which are used for the production ofacetic acid and bacterial SCP as described in more detail in thefollowing.

The chemical product of the fermentation can be used as an intermediatefor ethanol production by the following route (illustrated usingglucose) as described in more detail in U.S. patent application Ser. No.09/720,930 to make an integrated ethanol production process:

$\begin{matrix}{{Fermentation}\text{:}} & {\mspace{200mu} \left. {Glucose}\mspace{14mu}\Rightarrow{3\mspace{14mu} {Acetic}\mspace{14mu} {Acid}} \right.} \\{{Esterification}\text{:}} & \left. {{3\mspace{14mu} {Acetic}\mspace{14mu} {Acid}} + {3\mspace{14mu} {Ethanol}}}\mspace{14mu}\Rightarrow{{3\mspace{14mu} {Ethyl}\mspace{14mu} {Acetate}} + {3\mspace{11mu} H_{2}O}} \right. \\{{Hydrogenation}\text{:}} & \underset{\_}{\left. {{3\mspace{14mu} {Ethyl}\mspace{14mu} {Acetate}} + {6\mspace{14mu} H_{2}}}\mspace{45mu}\Rightarrow{6\mspace{14mu} {Ethanol}} \right.\mspace{175mu}} \\{{Overall}\text{:}} & {\mspace{85mu} \left. {{Glucose} + {6\mspace{14mu} H_{2}}}\mspace{56mu}\Rightarrow{{3\mspace{14mu} {Ethanol}} + {3\mspace{11mu} H_{2}O}} \right.}\end{matrix}$

Each step can be done at high yield, so nearly every mole of acetateproduced in the fermentation ends up as a mole of ethanol. In otherwords, each C6 sugar produces 3 moles of ethanol in our indirect route.By comparison, direct fermentation routes only produce 2 moles ofethanol for each C6 sugar. This 50% improvement in sugar utilization hasa dramatic effect on the economics of bioethanol production.

Alternatively the process in the current invention can utilize part ofthe ethanol produced in the conventional ethanol process and produceethyl acetate as a final product for sale as an additional product fromthe plant using the esterification process described in more detail inU.S. patent application Ser. No. 09/720,930 to make an integratedethanol production process.

This integrated process for the production of ethanol by dry milling ofcorn provides improved fiber utilization, upgrading of coproducts andthe production of a higher value new product compared to conventionalethanol production based on the dry milling of corn and utilizes theexisting physical assets of the original ethanol production plant to alarge extent. There are a number of features of the acetogenicfermentation, which make it particularly suited to corn fiberutilization in a dry mill.

Preparation of Bottoms Fraction as Fermentation Substrate

In a conventional corn dry milling ethanol production process, after thefermentation is conducted, the ethanol is distilled and recovered. Theremaining fermentation medium is referred to as a “bottoms fraction” or“beer bottoms.” In accordance with the present process, the bottomsfraction is used as a carbohydrate and nitrogen source in the processbroadly described in U.S. patent application Ser. No. 09/720,930.

The utilization of corn fiber in the bottoms fraction requires thehydrolysis of the fiber to monomeric sugars. This can be accomplished intwo stages, a pretreatment step and an enzymatic hydrolysis step.

The simplest pretreatment approach, and one that is suitable for cornfiber is the use of hot water at a moderately high temperature. Suchtreatments are well know in the art. For example, see Weil et al., Appl.Biochem. And Biotech., 73:1-17 (1998).

Prior to the pretreatment step, the pH can be adjusted to preventdegradation of the sugars by acid production. For example, the pH can beadjusted to between about 5 and about 7. Any suitable base can be usedfor pH adjustment, including without limitation potassium hydroxide.

The further hydrolysis of corn fiber after the pretreatment step ispreferably an enzymatic hydrolysis step. The enzymatic hydrolysis stepcan be conducted with a mixture of enzymes in combination, includingxylanase and cellulase. There are several reasons why the presentprocess is well suited to enzymatic hydrolysis on an economic basis. Theraw material stream required for the acetogenic fermentation is verydilute. This means that the stream subjected to enzymatic hydrolysis canbe made very dilute so as not as limited by feedback inhibition. Thedesired product hydrolysate can be about 3.5% fermentable sugars. Thetotal fermentable fraction of the resulting hydrolyzed bottoms fraction(i.e., carbohydrate plus nitrogen source) is preferably less than about6% by weight and more preferably less than about 5% by weight. Theconcentration of the hydrolyzed bottoms fraction can be controlledsimply by dilution of bottoms fraction from the yeast fermentation. Theacetogenic organisms can also use dimer sugars such as cellobiose. Theuse of mixtures of enzymes can reduce the total overall loading. Thehigh yield from the raw material in the fermentation reduces the totalenzyme cost in terms of cost per unit of product.

The resulting hydrolyzed bottoms fraction is then subjected to asolid/liquid separation and the liquid fraction (i.e., liquidhydrolysate) is used as a substrate in a subsequent fermentation, asdescribed below. The solids fraction is typically dried and can be driedwith the same type of drier that is used to dry DDGS, thereby utilizingequipment from existing corn dry milling ethanol production facilities.The solids fraction from the hydrolyzed bottoms fraction is a highprotein byproduct that includes the majority of the protein and thelipid fraction from the corn. DDGS typically has a protein content ofabout 28% by weight on a dry weight basis. By contrast, the high proteinbyproduct of the present invention has a protein of at least about 35%by weight, more preferably at least about 45% by weight and morepreferably at least about 50% by weight. The dried solids fraction ofthe present invention, while significantly higher in protein content istypically about one-half the volume of DDGS prepared from the samevolume of corn preparation.

In an alternative embodiment, the protein fraction of the corn in thebottoms fraction is hydrolyzed to amino acids and small peptides byprotease enzymes. This protein content thereby becomes available forconversion to bacterial single cell protein (SCP) in subsequentfermentation. SCP has a better amino acid profile that the original cornproteins since the bacteria can produce essential amino acids they needfrom the amino acids in the corn. The amino acid profile of bacterialSCP allows the marketing of the coproduct into an even wider range ofmarkets that corn gluten meal, including carnivorous fish, for example.Thus this alternative embodiment reforms the protein content of the corninto a higher value SCP. In this alternative process in which SCP isproduced, a protease enzyme is utilized in addition to the xylanase andcellulase enzymes during hydrolysis.

The invention involves the use of acetogenic bacteria. This class ofbacteria was first isolated in the 1930's and it is a group of obligateanaerobic bacteria that can use the acetyl-CoA pathway to fix carbondioxide, resulting in the reductive synthesis of acetic acid or theincorporation of carbon dioxide into cellular materials via acetyl-CoA.Habitats of these bacteria are sewers, sediments, termite guts, rumens,and the intestinal tracts of monogastrics including humans.Pathogenicity is rare.

The acetogenic bacteria include members in the Clostridium,Acetobacterium, Peptostreptococcus, Sporomusa and a couple of otherlesser known species. Acetogens can be further characterized as beingeither homoacetogens or heteroacetogens depending upon whether aceticacid is the only major metabolic product. By far the most work to datehas been done with C. thermoaceticum. C. thermoaceticum is a gramvariable, spore forming, thermophilic homoacetogen originally isolatedfrom horse manure.

These organisms metabolize glucose to pyruvate using the Embden-Meyerhofglycolytic pathway. Lactic acid is also metabolized by first convertingit back to pyruvate.

Pyruvate is decarboxylated and then oxidized to acetate with theconcurrent production of ATP. The main distinguishing feature ofacetogenic bacteria is that the CO₂ produced in the decarboxylation stepis not released to the environment. Instead, the acetyl-CoA pathway isused to fix the CO₂ and make an additional mole of acetic acid. Thus,for glucose fermentation the overall stoichiometry is:

Glucose→3 Acetic Acid

Two of the acetic acid molecules are made from oxidation of glucose, thethird is made by the reduction of the carbon dioxide byproduct ofglucose oxidation. Ignoring the effects of cell mass production, thehomoacetogens are capable of converting glucose into acetic acid at 100%theoretical mass yield.

A very important feature is the ability of the organisms to use a verywide range of raw material substrates. These include materials with awide range of carbon numbers.

Five carbon sugars like xylose, a major monomer of hemicellulose, areconverted to fructose-6-phosphate and glyceraldehyde-3-phosphate via thepentose phosphate pathway. Both of these intermediates are then passedto glycolysis to produce pyruvate, followed by conversion of pyruvate toacetate using the oxidative and acetyl-CoA pathways. While notreflective of the elementary reactions involved, the overallstoichiometry for xylose fermentation by acetogens is:

2 Xylose→5 Acetic Acid

thus homoacetogens are also capable of converting xylose into aceticacid at near 100% theoretical mass yield. This an important featuresince current R&D efforts for improving the value of fiber are focusedon hydrolysis of the cellulose and hemicellulose into a mixture of fiveand six carbon sugars. The natural ability of the acetogens tometabolize both five and six carbon sugars provides a distinct advantageover other strains considered for fermenting fiber hydrolyzates.

Overall the organisms can utilize a wide range of substrates (Drake,1994), including:

C3: glycerol, lactate

C5: xylose

C6: glucose, fructose

C12: sucrose, lactose, cellobiose, maltose

In addition, the organisms can utilize and tolerate acids, alcohols andaromatic monomers from the breakdown of lignin in biomass. Thesecombined features allow the organisms to utilize very complex rawmaterials from the hydrolysis of cellulosic material from corn fiber,corn stover (stalks), or other biomass. Often the complete hydrolysis ofcellulosic biomass, including corn fiber, combines acid hydrolysis withenzymatic hydrolysis. The breakdown products from acid hydrolysisproduce organic acids and aromatic monomers, which are inhibitory totraditional fermentation organisms such as yeast.

Recent efforts have shown that corn steep liquor is a satisfactorylow-cost media component that greatly improves cell growth rates (Bocket. al. 1997, Witjitra et. al. 1996, Balasubramanian et. al. 2001).

Several kinetic studies (Yang et. al. 1988, Wang et. al. 1978, Tang et.al. 1988) have been conducted to examine the effects of pH and acetatelevels on both cell growth and acid production. Unfortunately theorganisms are sensitive to low pH and product inhibition occurs at muchlower concentrations than with lactic acid bacteria. Optimal pH isaround 7 and maximum acetate tolerance is about 30-50 g/l in batchfermentation

There are other useful features of the fermentation from an industrialperspective. The fermentation takes place at a relatively hightemperature, e.g. 58° C., thus reducing the possibility of contaminationof the fermentation by stray organisms. The fermentation is anaerobicand thus there is no need for gas exchange. This greatly reduces themixing and gas compression requirement and eliminates another source ofpossible contamination.

The most important reason that acetogenic fermentations have not beenutilized at an industrial scale is the limitation of productconcentration in the fermentation broth by product inhibition. Aceticacid and acetate salts are very toxic to the fermentation organisms. Atlow concentrations, about 3-4%, the product stops the fermentation. Thusthe product from the fermentation is a dilute acetate salt solution suchas calcium acetate. This limitation is overcome by the reactiveseparation process described below.

Fermentation Process

The overall purpose of the fermentation part of the current invention isto convert the fermentable carbohydrates and amino acids into aceticacid and single cell bacterial protein. In a preferred embodiment a twostep fermentation process is used. The first step uses ahomofermentative lactic acid bacteria to convert the bulk of thefermentable sugars into lactic acid and single cell protein. The secondstep uses a homofermentative acetogenic bacteria to convert lactic acidand residual carbohydrates into acetic acid.

The lactic acid fermentation step uses a homofermentative lactic acidbacteria such as Lactobacillus casei to convert the fermentable sugarsinto lactic acid. Lactic acid bacteria are gram-positive, non-sporeforming, aerotolerant anaerobes. These bacterial are found in the mouthsand intestinal tracts of most warm blooded animals including humans.None are pathogenic and many are approved by the United States FDA asviable organisms for use in direct-fed microbials for animal feeds.Viable cultures are also present in many yogurts consumed by humans.

As shown in FIG. 3, lactic acid is the sole metabolic product forhomofermentative strains. Glucose is metabolized to pyruvate using theregular Embden-Meyerhof glycolytic pathway. Pyruvate is converted tolactic acid in a single NAD coupled step. Most lactic acid bacteria aremesophilic with optimal temperatures for growth between 35 to 45 C. Cellgrowth is pH sensitive with optimal pH around 6.0. Product inhibitionbegins to affect the kinetics of cell growth and acid production atlactic acid levels above 4 wt %. Complete inhibition of growth occursaround 7 wt % while complete inhibition of acid production occurs around10-12 wt %.

The feed to the fermentation is very dilute in carbohydrates with onlyabout 5 wt % fermentable sugars. A single stage continuous stirred tankreactor (CSTR) type fermentor is appropriate for this step. However, anysuitable bioreactor can be used, including batch, fed-batch, cellrecycle and multi-step CSTR. The low carbohydrate concentration in thefeed will limit the effects of product inhibition on the cell growth andacid production kinetics, thus 90+% conversion of the dextrose withabout 18-24 hour residence times is possible. Most homofermentativestrains will readily metabolize a range of substrate sugars. It isadvantageous to combine the lactic acid fermentation with the subsequentacetic acid fermentation in such a manner so as to utilize all of thesugars.

In contrast to many industrial lactic acid fermentations, the currentinvention may be operated in a mode in which the fermentation iscarbohydrate limited rather than nitrogen limited. Thus biomassproduction is maximized by keeping most of the fermentation in thegrowth associated state and ensuring that sufficient nitrogen isavailable for growth. For any growth associated fermentation the biomassyields are typically about 10.5 g per mole of ATP produced. Since lacticacid fermentations produce a net of 2 moles of ATP per mole of glucose,the biomass yield will be around 2 (10.5/180)=0.12 g per g of glucose.By stoichiometry, the remaining 0.88 g of glucose are converted into0.88 grams of lactic acid.

The efficient production of biomass as single cell protein is animportant part of this invention. In contrast to the production ofsingle cell protein historically, the use of an anaerobichomofermentative fermentation is very advantageous. This is because allof the energy production of the organism comes from the production ofthe desired metabolite whether lactic acid or acetic acid. This meansthat there is no wasted byproduct CO₂ as is the case in aerobicfermentations. In addition, because of the lack of production of CO₂,the heat produced by the fermentation is also minimized. Therefore theutilization of energy contained in the raw material carbohydrates ismaximized toward the production of valuable single cell protein orlactic and acetic acid. The traditional yeast fermentation, in additionto wasting mass as CO₂, also requires the removal of heat.

The fermented broth from the first fermentation step is clarified usinga centrifuge. The concentrate contains the lactic acid bacteria and issent to single cell protein recovery. The amount of single cell proteinproduced is related to the amount of nitrogen in the form of hydrolyzedproteins as amino acid and peptides that is supplied to the fermentationin the medium. This can range from a very small amount, but not zero, aslactic acid bacteria require some complex nitrogen sources, such as 1%up to about 15% overall yield of single cell protein based on the totalnitrogen plus carbohydrate in the medium. It is a feature of theinvention that the production of single cell protein can be controlledover a wide range. The single cell protein can be processed by anysuitable means, such as spray drying, to produce a salable product.

Another important feature of the current invention is the production ofa single cell protein which is enhanced in value as an animal feedingredient. The single cell protein from the lactic acid fermentationhas these features. It has a high protein concentration of about 70%,depending on the strain of organism and the specific conditions of thefermentation. It has a good amino acid profile. That is, it contains ahigh percentage of so called essential amino acids comprising, forexample, lysine, methionine, isoleucine, tryptophan, and threonine. Thecombined percentage of these amino acids in lactic acid bacteria isabout 10.5%, compared to corn protein which has about 1% of the totalcorn kernel. The protein composition of corn depends on the fraction ofthe corn considered. Corn gluten meal, for example, has about 7.5%, butcorn gluten feed has about 2.5% of essential amino acids. This enhancedamino acid composition is directly related to the value of the proteinas an animal feed ingredient.

In a preferred embodiment, the current invention can produce single cellprotein at high efficiency and with high value. The centrate, from theseparation of the lactic acid bacteria from the fermentation broth ofthe first fermentation, is fed to a second fermentor where the lactateis converted into acetate using an acetogenic bacteria. Lactate can be apreferred substrate for acetogenic bacteria in many of their naturalenvironments. The rate of fermentation and yield on lactate substratecan be very high, e.g., over 98% yield of acetate from lactate.

Incomplete removal of the lactic acid bacteria is typically acceptablesince the acetic acid fermentation typically uses a thermophilic strainand the second fermentation is done at a higher temperature.Contamination of the acetic acid fermentation with a mesophilic lacticacid bacteria is typically not an issue since the lactic acid bacteriatypically cannot grow at these higher temperatures. Also, near completeconversion of the glucose is expected in the first fermentor, so thelactic acid bacteria which do happen to bleed through the centrifugeinto the second fermentor will not have a carbohydrate source.

The acetogenic bacteria have been known and studied since the 1930′s.Drake, H. L. (editor), Acetogenesis, Chapman & Hall, 1994, gives a goodoverview of the field. The acetogenic bacteria include members in theClostridium, Acetobacterium, Peptostreptococcus and other lesser knownspecies. The habitats of these bacteria are: sewers, anaerobic digestersat municipal waste treatment plants, natural sediments, termite guts,rumens, and intestinal tracts of non-ruminants including humans.Pathogenicity is rare. All of these organism are strict anaerobes, whichmeans that contact with oxygen is often fatal to the microorganism.Clostridium are spore formers. Spores are resistant to manysterilization techniques and special procedures have been establishedfor handling spore-forming bacteria. The Acetobacterium andPeptostreptococcus species are not spore formers.

FIG. 4 is a simplified sketch of the metabolic pathways used by mostacetogenic bacteria. The organism metabolizes glucose to pyruvate usingthe normal Embden-Meyerhof glycolytic pathway. Lactic acid is alsometabolized by first converting it back to pyruvate. From pyruvate, theorganism makes acetic acid and carbon dioxide using the regularoxidation pathways. The main distinguishing feature of acetogenicbacteria is that the CO₂ produced in this oxidation step is not releasedto the environment. Instead, the acetogenic bacteria have a metabolicpathway which will fix the CO₂ and make an additional mole of aceticacid.

The novel acetogenic pathway provides three functions for the organism:

1. Like all anaerobes, a terminal electron acceptor other than oxygen isrequired to balance the redox reactions of metabolism. In this case, thereduction of carbon dioxide acts as the electron sink.

2. Cellular energy (i.e. ATP) is produced from this pathway. Themetabolic pathways for conversion of one mole of glucose into two molesof acetic acid and two moles of carbon dioxide produce four ATP per moleof glucose consumed. Addition of the acetogenic pathways creates anotheracetic acid molecule from the carbon dioxide and increases the ATP yieldto 4 B 6 ATP per mole of glucose. The additional ATP are not madedirectly from the substrate-level phosporylation but are made in otherprocesses such as the electron transport chain and from ion pumpslocated in the cell membranes. The exact amount of ATP produced from thesecondary sources varies from strain to strain and is also dependentupon the cell environment.

3. Carbon dioxide can be converted into cellular carbon needed forgrowth using the cell's anabolic pathways, even when common carbonsources such as glucose are not available.

Some acetogens will produce other organic acids such as formic,propionic, succinic, etc. in addition to acetic acid. These organismsare described as heterofermentative as opposed to the homofermentativeorganisms which only produce acetic acid. The heterofermentativepathways represent a potential yield loss in the current invention, andproper strain selection and elucidation of the factors which cause theformation of these other organic acids will minimize the impact.

By far, most work to date has been with the Clostridium strains. Many ofthese strains are thermophilic with optimal temperatures for growtharound 60 C. Several kinetic studies (Yang, S. T., Tang, I. C., Okos, M.R., “Kinetics and Mathematical Modeling of Homoacetic Fermentation ofLactate By Clostridium formicoaceticum”, Biotechnology andBioengineering, vol. 32, p. 797-802, 1988, Wang, D. I., Fleishchaker, R.J.; Wang, G. Y., “A Novel Route to the Production of Acetic Acid ByFermentation”, AIChE Symposium Series-Biochemical Engineering: RenewableSources, No. 181, vol. 74, p. 105-110, 1978; and Tang, I. C., Yang, S.T., Okos, M. R., “Acetic Acid Production from Whey Lactose by theCo-culture of Streptococcus lactis and Clostridium formicoaceticum”,Applied Microbiology and Biotechnology, vol. 28, p. 138-143, 1988, whichare incorporated herein by reference in their entirety) have beenconducted to examine the effects of pH and acetate levels on both cellgrowth and acid production. These organism are sensitive to low pH andproduct inhibition occurs at much lower concentrations than in lacticacid bacteria. Optimal pH is around 7 and maximum acetate tolerance isonly about 30 g/l in batch fermentation.

A one or two stage CSTR fermentor design is typically appropriate forthe second fermentation step. However, any suitable bioreactor can beused, including batch, fed-batch, cell recycle, and multi-step CSTR. Incontrast to the first fermentation step, the acetic acid fermentation isnitrogen limited rather than carbohydrate limited. Yield of acetic acidfrom lactic acid can be greater than 85% of theoretical.

In one embodiment, the broth from the second fermentation step isprepared for the second part of the current invention which is thechemical conversion. As an example, the broth is clarified with acombination of a centrifuge and a microfilter. The centrifuge removesthe bulk of the biomass and reduces the size of the downstreammicrofilter by reducing its load. The microfilter permeate is sent to ananofiltration unit. The microfilter acts as a prefilter for thenanofiltration unit. The nanofiltration unit removes proteins,unconverted sugars, etc. which have molecular weights above about 300.The nanofiltration unit removes the bulk of the impurities in theacetate broth and produces a water white permeate that can be sent todownstream processes.

The concentrates from the centrifuge, microfilter and nanofilter may beprocessed to recover values useful in the single cell protein orrecycled to one of the fermentation steps. Alternatively, they may bedisposed of in any acceptable manner such as composting or incineration.

Although a preferred embodiment of the current invention utilizes twofermentation steps and the production of single cell protein, this isnot required in the most general case. A suitable medium for the aceticacid fermentation alone may be provided. Although single cell proteinmay not be produced, the increased yield form the carbohydrate sourcewill still provide an important advantage for the current invention.

The key feature of the fermentation step is therefore the conversion ofcarbohydrate from any source into acetic acid.

Acidification and Esterification

In the next step of the invention, the acetic acid or acetate producedin the fermentation is converted to an ester of acetic acid, preferablymethyl or ethyl ester and more preferably ethyl ester. Any suitableprocess that will convert the acetic acid or acetate salt to the esteris acceptable as part of this invention.

Acetic acid is a weak organic acid with pKa=4.76. If the fermentation isconducted at near neutral pH (i.e. pH=7.0), the product of thefermentation will actually largely be an acetate salt rather than theacid. In the fermentation, any suitable base can be used to neutralizethe fermentation. The preferred neutralizing agent is Ca(OH)₂, which canbe supplied by CaO (lime) or calcium carbonate (CaCO₃) which can berecycled from later in the process. Other neutralizing agents can beused, such as NaOH or NH₄OH, as determined by the conditions required bythe fermentation organism. However, even the acetate salt is inhibitoryand the maximum concentration of acetate is usually limited to about 5%in the fermentation broth.

Thus, there are two problems in the recovery of acetic acid salts from asolution such as a fermentation broth. The acetate salt must usually beconverted to the acid, and the acid must be removed from the dilutesolution in water. In addition it is desirable to recycle the base usedto neutralize the fermentation to reduce costs and avoid potentialenvironmental impact.

The most typical route is the sequential acidification of the salt toproduce acetic acid and then the subsequent recovery of the acid. Evenafter the salt is converted to a dilute acid solution, there is stillthe need to recover the product from the water. Many different processapproaches have been proposed to recover such dilute solutions. Sinceacetic acid has a higher boiling point than water, the bulk of thewater, about 95% of the broth, must be distilled away from the aceticacid to recover the acid if simple distillation is used. Alternatively,some more complex process may be used to recover the acetic acid,usually in conjunction with solvent extraction. However this line ofresearch, that is, acidification with subsequent recovery from thedilute solution, has not overcome the economic limitations of the aceticacid fermentation process to make it competitive with the synthesis gasbased route. Therefore, all industrial acetic acid is currently madefrom synthesis gas derived from coal, petroleum or natural gas.

A number of methods have been proposed to acidify the acetic acid saltsolution. One method is the reaction of the acetate salt with a strongacid such as sulfuric acid to form acetic acid (HAc) and calcium sulfate(CaSO₄). The CaSO₄ precipitates and is easily separated from the aceticacid solution. However, this method requires the consumption of acid andbase and produces a byproduct waste salt that may become anenvironmental burden. Another method is bipolar electrodialysis thatsplits the salt into an acid and base (this does not work well with Casalts, but one could substitute Na in this case). Other routes toproduce dilute acetic acid from the salt are well known.

Reaction of a carboxylic acid salt with an amine and CO₂ with theprecipitation of CaCO₃ and the formation of an acid amine complex thatcan be extracted and thermally regenerated has also been proposed, asshown by U.S. Pat. No. 4,405,717, which is incorporated herein byreference in its entirety.

U.S. Pat. No. 4,282,323, which is incorporated herein by reference inits entirety, discloses a process to acidify acetate salts using CO₂ ina number of ways. In the referenced patent the acetic acid formed isremoved by a solvent to a separate phase.

Esterification of acetic acid to form ethyl acetate is a well understoodreaction:

Esterification is typically performed in the liquid phase. Theequilibrium constant for this reaction is 4.0 and is nearly independentof temperature. Acid catalysts for the reaction include: strong Bronstedacids such as sulfuric acid and methane sulfonic acid, acidic ionexchange resins, zeolites, and a number of other materials, includingcarbonic acid formed by the dissolution of CO₂ in water. The reactionrate is influenced by the type and concentration of catalyst, thereaction temperature, and the degree of departure from equilibrium.

Alternative routes exist that attempt to avoid the separateacidification and esterification steps. A carboxylic acid salt may bereacted directly with an alcohol such as ethanol to produce the esterdirectly. An intermediate step may be inserted to convert the Ca salt toan ammonia salt. In this step the dilute Ca(Ac)₂ is reacted with NH3 andCO₂ to form NH₄Ac and CaCO₃ which precipitates. The ammonia salt ofacetic acid may then be esterified directly as shown by U.S. Pat. No.2,565,487, which is incorporated herein by reference in its entirety.

Preferred Approach

The preferred approach is to combine chemical and phase changeoperations into a new efficient process to directly produce a volatileester of acetic acid and distill the ester away from the broth.

The three parts are:

1) Acidification of the fermentation broth with CO₂ at low or nearlyatmospheric pressure to produce acetic acid and precipitate CaCO₃ whichcan be recycled directly to the fermentation as the base;

2) Simultaneous esterification of the formed acetic acid with analcohol, such as methyl or ethyl alcohol, to form a volatile ester, and

3) Reactive distillation to push the acidification and esterificationequilibria to high conversion.

Since esterification is an equilibrium reaction, high conversion can beobtained by driving the reaction to the right with continuous removal ofone or more products. Reactive distillation similar to that developed byChronopol for lactide synthesis (See U.S. Pat. No. 5,750,732, which isincorporated herein by reference in its entirety) and by EastmanChemical for methyl acetate production (see U.S. Pat. Nos. 4,939,294 and4,435,595 and Agreda, V. H., Partin, L. R., Heise, W. H., “High-PurityMethyl Acetate Via Reactive Distillation”, Chemical EngineeringProgress, p. 40-46, February 1990, which are incorporated herein byreference in their entirety) is an economically attractive method. U.S.Pat. No. 5,599,976, which is incorporated herein by reference in itsentirety, discloses the conversion of very dilute acetic acid to theester in a continuous reactive distillation process. Xu and Chaung (Xu,Z. P, Chuang, K. T., “Kinetics of Acetic Acid Esterification over IonExchange Catalysts”, Can. J. Chem. Eng., pp. 493-500, Vol. 74, 1996)show that reactive distillation to produce the ester of acetic acid fromdilute solution is the preferred method to remove acetic acid from verydilute solutions, as are produced in the current invention. In thisconcept, the acetic acid flows in a counter current fashion to theesterifying ethanol in a distillation column. In the current invention,ethyl acetate is more volatile than acetic acid so the ethyl acetate isdistilled away from the liquid mixture and the esterification reactionis pushed to the right, thus enabling high conversions in a singlevessel. The process proposed here goes beyond these examples in that itscombines simultaneous acidification with the reactive distillationesterification. All of the cited processes start with acetic acid (orlactic acid in the Chronopol case) and not a salt.

The net effect of the reactive distillation process, the preferredroute, is to remove the acetic acid from the dilute solution withoutvaporizing the water which forms the bulk of the stream.

In addition, the use of CO₂ as the preferred acidifying agent with theprecipitation of CaCO₃ allows the recycle of the neutralizing agent tothe fermentation without the consumption of chemicals. The CaCO₃ can beused directly in the fermentation or can be converted first to CaO bycalcination.

The reactive distillation process is shown in FIG. 5.

Reaction section: The raw material, a dilute (5%) solution of calciumacetate 410 (Ca(Ac)₂) in water 414 is mixed with ethanol 418 and fed tothe column 422 at the top of the reaction section 424. CO₂ 420 is fed tothe column 422 at the bottom of the reaction section 424. Thesimultaneous reaction of CO₂ 420 with Ca(Ac)₂ 410 and ethanol 418 takesplace in the reaction zone 424 in the center section of the column 422with the formation of CaCO₃ 428 and ethyl acetate (EtAc) 432.

CO₂(g)+H₂O−>H2CO3

Ca(Ac)₂+H₂CO₃−>CaCO₃(s)+2HAc

2HAc+2EtOH−>2EtAc

The most volatile component in the reaction mixture is the ethylacetate/water/ethanol azeotrope 436. The azeotrope composition is 82.6%ethyl acetate, 9% water and 8.4% ethanol and has a normal boiling pointof 70.2° C. The azeotrope 436 is removed from the reaction mixture byvaporization along with some EtOH and water. The bottom product from thereaction zone is a water and ethanol solution containing the suspendedCaCO₃ flowing to the stripping section.

Separation Section: In the upper separation zone 450 the azeotrope isseparated from the ethanol and water also vaporized from the reactionmixture. The ethanol water mixture 454 is recycled to the reaction zone424 and the overhead product is the azeotrope 436. The CO₂ is separatedfrom the overhead condensate and recycled to the column with makeup CO₂.The azeotrope can be broken by the addition of water, which causes aphase separation, with the water and ethanol rich phase returned to theappropriate point in the reactive distillation column (not shown).

Stripping Section: Since excess ethanol is used to favor the forwardesterification reaction in the reaction section, the stripping section458 returns the excess ethanol to the reaction zone. In the strippingsection 458, the ethanol is removed from the CaCO₃-containing waterstream which is discharged from the column 422 and separated by a simpleliquid /solid separation 462, such as centrifugation or filtration, intothe solid base 466 for recycle and water 470.

The net effect of the reactive distillation process is to recover theacetic acid from the dilute salt solution thereby producing a relativelyconcentrated product stream at the top and without vaporizing the waterthat forms the bulk of the stream. The integration of the three sectionsreduces the energy requirement. The simultaneous removal of the productester shifts the esterification equilibrium and leads to higherconversion in a short time.

It is unusual to handle precipitates in a distillation system. However,in this case the precipitation reaction occurs in the bulk phase and isnot due to the concentration of the solution at a heat transfer surface,a common type of fouling. Ethanol beer stills in the corn dry millingethanol industry typically handle solids loading in the strippingsection through the use of trays with simple construction and largeopenings. Alternatively, it would be possible to operate the reactionsection in other configurations, such as a series of stirred tanks witha common vapor manifold, to simulate the column reaction section.

The successful development of a low cost, low energy, integratedacidification, esterification and purification process for ethyl acetatewould potentially allow the economic production on an industrial scaleof major chemicals from renewable resources, which are now produced fromnon-renewable resources.

One major benefit of using renewable resources is the reduction of CO₂production with the replacement of fossil raw materials. There would bea benefit to the U.S. economy from the replacement of imported petroleumwith domestic renewable resources. The use of agricultural commoditiesto produce chemicals and liquid fuels without subsidy has importantbenefits to the farm community in terms of product demand and stablemarkets and reduces the cost of U.S. government subsidies.

Hydrogenation

The third major step in the invention is the conversion of the ester ofacetic acid into two alcohols by hydrogenation. The hydrogenation ofesters to produce alcohols is a well-known reaction.

U.S. Pat. Nos. 2,782,243, 4,113,662, 4,454,358, and 4,497,967, which areincorporated herein by reference in their entirety, disclose processesfor the hydrogenation of esters of acetic acid to ethanol.

For the particular case at hand, hydrogenation can be performed ineither the liquid phase or the gas phase. Any suitable hydrogenationprocess can be used. This reaction is also an equilibrium reaction. Thereaction can be driven to the right by using high partial pressures ofhydrogen. Typical reaction conditions are 150-250° C. and 500-3000 psidepending upon the desired conversion and selectivity. The reaction canbe catalyzed by any suitable hydrogenation catalysts, such as copperchromite, nickel, Raney nickel, ruthenium, and platinum. A copperchromite, nickel, or Raney nickel catalyst is preferred for thehydrogenation since these catalysts are not poisoned by water. In theliquid phase process, an alcohol such as ethanol is a good solvent.

In the gas phase process, the ethyl acetate feed is vaporized and fed tothe hydrogenation reactor with an excess of hydrogen. After passingthrough the bed, the vapors are cooled and flashed into a low pressureknockout drum. The hydrogen rich vapor phase is recycled back to thereactor. The liquid phase is distilled to remove residual water andunreacted ethyl acetate. The water is not made by the hydrogenationchemistry; its source is the liquid-liquid equilibrium level present inthe upstream reflux drum of the reactive distillation column.

Another distillation column may be needed as a final polishing step,depending upon the nature and quantities of side products from theesterification and hydrogenation units.

The preferred ester is ethyl acetate, as it avoids the introduction of asecond compound into the process which must be purified away from theproduct stream.

The water stripper collects water streams from the acidification,esterification, and hydrogenation units. The water is steam stripped torecover solvent values, then the water is sent to final treatment anddischarge or recycled to the fermentation section.

Many potential sources of hydrogen for use in the present inventionexist. Any suitable hydrogen source can be used that produces hydrogenof sufficient purity for the hydrogenation reaction and that will notpoison the catalyst. Raw materials for hydrogen production include waterfrom which hydrogen can be produced by electrolysis. Many fossil andrenewable organic feedstocks can also be used. If a fossil feedstock isused, such as methane from natural gas, some CO₂ will be produced alongwith the hydrogen. However, if a renewable feedstock is used then theCO₂ production will be neutral to the environment. For example,feedstocks which contain carbon and hydrogen at the molecular level canbe used to produce hydrogen. Wood chips, sawdust, municipal wastes,recycled paper, wastes from the pulp and paper industry, solidagricultural wastes from animal and/or crop production are all examplesof renewable feedstocks that can be used for hydrogen production, e.g.,using gasification technology.

Steam reforming of methane to produce hydrogen is a well know process.Natural gas and water are reacted in a steam reformer to form hydrogenand carbon dioxide. Other methods to produce hydrogen (partial oxidationof hydrocarbons, partial oxidation of coal, water electrolysis, etc.)could also be used. Where pure oxygen is available, such as in afenceline operation with an air separations plant, the partial oxidationprocesses can be economically viable. Where inexpensive sources ofelectricity are available, electrolysis can be viable.

Another advantage of the current invention, compared to prior arttechnology for ethanol production, is the heat balance in the process.In the current invention, if hydrogen is made by steam reforming onsite, excess heat is available at high temperature and in an integratedplant due to the hydrogenation reaction of the ester being a highlyexothermic process. Therefore, the overall process is highly energyefficient. In addition, none of the carbohydrate raw material is wastedas CO₂ with the attendant generation of heat, which must be wasted tocooling water.

While various embodiments of the present invention have been describedin detail, it is apparent that modifications and adaptations of thoseembodiments will occur to those skilled in the art. However, it is to beexpressly understood that such modifications and adaptations are withinthe spirit and scope of the present invention.

1. A process for upgrading the unfermented fraction of corn from a yeastfermentation based corn dry milling ethanol process, comprising: a.hydrolyzing an unfermented fraction of corn from a yeast fermentationbased corn dry milling ethanol process; b. separating the hydrolyzedunfermented fraction into a solids fraction and a liquid hydrolyzate; c.culturing a microorganism in the liquid hydrolyzate; and d. wherein thesolids fraction has an increased protein content compared to theunfermented fraction.
 2. The process, as claimed in claim 1, wherein theprotein content of the solids fraction is at least about 35% protein ona dry weight basis.
 3. The process, as claimed in claim 1, wherein theprotein content of the solids fraction is at least about 45% protein ona dry weight basis.
 4. The process, as claimed in claim 1, wherein theprotein content of the solids fraction is at least about 50% protein ona dry weight basis.
 5. The process, as claimed in claim 1, wherein thestep of hydrolyzing the unfermented fraction comprises enzymatichydrolysis.
 6. The process, as claimed in claim 1, wherein the step ofhydrolyzing comprises contacting the unfermented fraction with axylanase.
 7. The process, as claimed in claim 1, wherein the step ofhydrolyzing comprises contacting the unfermented fraction with acellulase.
 8. The process, as claimed in claim 1, further comprising thestep of drying the solids fraction.
 9. The process, as claimed in claim1, wherein the microorganism is homofermentative.
 10. The process, asclaimed in claim 1, wherein the step of culturing produces acetate,acetic acid, or mixtures thereof.
 11. The process, as claimed in claim10, further comprising chemically converting the acetate, acetic acid ormixtures thereof to an ester of acetic acid.
 12. The process, as claimedin claim 1, wherein the unfermented fraction comprises a bottomsfraction produced by distilling ethanol from the yeast fermentation. 13.A process for upgrading the unfermented fraction of corn from a yeastfermentation based corn dry milling ethanol process, comprising: a.hydrolyzing an unfermented fraction of corn from a yeast fermentationbased corn dry milling ethanol process, wherein the step of hydrolyzingcomprises enzymatic hydrolysis; b. separating the hydrolyzed unfermentedfraction into a solids fraction, comprising at least about 35% proteinon a dry weight basis and a liquid hydrolyzate; c. culturing amicroorganism in the liquid hydrolyzate; and d. wherein the solidsfraction has an increased protein content compared to the unfermentedfraction.
 14. The process, as claimed in claim 13, wherein the proteinof the solids fraction is at least about 45% protein on a dry weightbasis.
 15. The process, as claimed in claim 13, wherein the protein ofthe solids fraction is at least about 50% protein on a dry weight basis.16. The process, as claimed in claim 13, wherein the step of hydrolyzingcomprises contacting the unfermented fraction with a xylanase.
 17. Theprocess, as claimed in claim 13, wherein the step of hydrolyzingcomprises contacting the unfermented fraction with a cellulase.
 18. Theprocess, as claimed in claim 13, further comprising the step of dryingthe solids fraction.
 19. The process, as claimed in claim 13, whereinthe microorganism is homofermentative.
 20. The process, as claimed inclaim 13, wherein the step of culturing produces acetate, acetic acid,or mixtures thereof.
 21. The process, as claimed in claim 20, furthercomprising chemically converting the acetate, acetic acid or mixturesthereof to an ester of acetic acid.
 22. The process, as claimed in claim13, wherein the unfermented fraction comprises a bottoms fractionproduced by distilling ethanol from the yeast fermentation.